1. Field of the Invention
The present invention relates to a viscous coupling clutch and more specifically to a viscous coupling clutch whose transmission torque is adjustable, when a first rotatable member is moved axially relative to a second rotatable member so that the overlap and gap conditions between first and second resistance cylinder assemblies change within a working chamber filled with a viscous fluid.
2. Description of the Prior Art
An example of viscous coupling clutches of overlapped resistance cylinder type whose transmission torque can be adjusted is disclosed in Japanese Published Unexamined (Kokai) Patent Appli. No. 62-106130, as shown in FIG. 1(A). In the drawing, a driving (input) side includes an input shaft 101, a slidable sleeve 102 spline coupled to the input shaft 101 so as to be slidable relative to the input shaft 101 in the axial direction thereof, a flange portion 103 fixed to or formed integral with the slidable sleeve 102, and a plurality of driving side resistance cylinders 104 of different diameters fixed to the flange portion 103 in coaxially and radially spaced positional relationship to each other. A driven (output) side includes a housing 105, a first side wall 106 fixed to one end of the housing 105 so as to be rotatable and axially slidable relative to the slidable sleeve 102, a second side wall 107 fixed to the other end of the housing 105 also so as to be rotatable and axially slidable relative to the slidable sleeve 102, and a plurality of driven side resistance cylinders 108 of different diameters fixed to the second side wall 107 in coaxially and radially spaced positional relationship to each other so as to be overlapped with and interposed between the driving side resistance cylinders 104 at small radial intervals. Further, the housing 105 is filled with a viscous fluid with a high viscosity.
Therefore, when the slidable sleeve 102 is moved in the axial direction thereof, since the driving side resistance cylinders 104 fixed to the slidable sleeve 102 are slid relative to the housing 105, it is possible to adjust the viscous coupling condition, that is, a torque generated by a shearing force between the two driving and driven side resistance cylinders 104 and 108.
In the above-mentioned prior-art viscous coupling clutch, the overlap condition of the two driving and driven side resistance cylinders 104 and 108 is changed in order to adjust the torque transmitted from the driving side to the driven side. In other words, a stroke of the slidable sleeve 102 is adjusted relative to the housing 105 to adjust the torque. Therefore, in order to obtain a sufficient adjustable range of the transmission torque between the two driving and driven sides, a relatively long stroke of the slidable sleeve 102 and therefore a relatively long axial length of the resistance cylinders 104 and 108 is inevitably required, thus resulting in a problem in that the size of the viscous coupling clutch increases and therefore the response speed of the clutch in response to a control signal decreases or a large energy is required for the actuator to improve the response speed.
Another example of viscous coupling clutches of overlapped resistance cylinder type whose transmission torque can be adjusted is disclosed in Japanese Published Unexamined (Kokai) Utility Model Appli. No. 62-32232, as shown in FIG. 1(B). This viscous coupling clutch is designed so as to be usable as a differential limiting device incorporated in a center differential gear for four-wheel drive vehicles. In the drawing, a first housing 111 is spline coupled to a front wheel driving shaft (not shown) of the center differential gear, and a second housing 112 is spline coupled to a rear wheel driving shaft (not shown) of the center differential gear so as to be rotatable relative to the first housing 111. In the same way, a working chamber 113 is formed between the first and second housings 111 and 112 and filled with a viscous fluid. Within the working chamber 113, a plurality of first resistance cylinders 114 of different diameters are fixed to the first housing 111 in coaxially and radially spaced positional relationship to each other, and a plurality of second resistance cylinders 115 of different diameters are fixed to the second housing 112 in coaxially and radially spaced positional relationship to each other so as to be overlapped with and interposed between the two first resistance cylinders 115 at small radial intervals. The feature of this viscous coupling clutch is to form the resistance cylinders 114 and 115 in such a way that a radial wall thickness of each resistance cylinder decreases gradually from the fixed end to the free end thereof.
When the vehicle travels straight on a paved road, since the first housing 111 coupled to a front wheel drive shaft and the second housing 112 coupled to a rear wheel drive shaft both rotate at the same speed, no differential limiting torque is generated between the two housing 111 and 112.
When the vehicle travels on a muddy road or turns, that is, when road surface and travelling conditions change, a difference in speed is produced between the two housings 111 and 112. Therefore, the viscous fluid within the working chamber 113 is sheared by the first and second resistance cylinders 114 and 115, so that a torque is transmitted from the first housing 111 to the second housing 112 or vice versa.
When the transmission torque between the two housings is required to be adjusted according to the necessity, an actuator 116 is activated to pivot a pivotal lever 117 so that the second housing 112 is moved toward or separated from the first housing 111. When the second housing 112 is adjustably moved relative to the first housing 111, since the radial gap t.sub.1 between the first and second resistance cylinders 114 and 115 and also the overlap condition of the two cylinders 114 and 114 are changed, the shearing resistance of the viscous fluid changes and therefore the transmission torque is adjusted. As described above, the differential limiting torque of the center differential gear is adjusted according to road and travelling conditions for providing a safe vehicle driving.
In the above-mentioned second prior-art viscous coupling clutch, however, since the radial cross-section of each of the first and second resistance cylinders 114 and 115 must be formed into a triangular shape so that the radial wall thickness thereof gradually decreases from the fixed end to the free end thereof, there exists another problem in that it is difficult to manufacture the above-mentioned resistance cylinders 114 and 115 with a triangular radial cross section and therefore the manufacturing cost thereof inevitably rises.